If you are in a lightbox and find you cannot delete or change images and you see a sync button on the toolbar, but no trashcan this means that the gallery is synced with a Dropbox folder.
When this is done you have to make deletions/changes in Dropbox, then re-sync the connection. We provided the sync facility for people who use Dropbox extensively in their workflow, this way multiple folders can be linked to Dropbox and with a single button press the portfolio can be updated.
However many people won't need this and only need to use Dropbox as a download source to provision Foliobook.
If you see the sync button on the toolbar, and this isn't what you need any longer, select the sync button and 'unlink' to disconnect from the Dropbox folder. Then you can manage the files locally in the lightbox again